A wide variety of heart valve prostheses have been developed which operate hemodynamically in unison with the pumping action of the heart. Some of the earliest valves included a ball and cage arrangement, and subsequent valves were developed utilizing a single disc or occluder that either floated, or pivoted along an eccentric axis, between open and closed positions. Subsequently, various streamlined designs were advocated so as to improve the performance of such valves. Examples of valves of this type are shown in U.S. Pat. Nos. 4,011,601, 4,425,670, and 4,484,365. At about the same time, prosthetic heart valves employing a pair of occluders were developed, and these valves are often referred to as bileaflet valves. In such an arrangement, both of the occluders are generally identical in shape and size and are designed to pivot about eccentric axes. U.S. Pat. Nos. 4,535,484 and 4,846,830 are representative of such bileaflet valves. Although the patent art is replete with different designs for prosthetic heart valves, the majority of such designs have never achieved commercial reality, often because particular advantages which the valve may have are accompanied by shortcomings that prevent their commercialization.
In general, a prosthetic valve should provide a relatively unobstructed, fairly wide open passageway when in the open position, so as to only minimally retard the flow of blood therethrough in the downstream direction, creating a relatively low pressure drop across the valve. In addition, the design should be such as to eliminate or minimize turbulence by delaying blood flow separation from the leaflet surfaces. Turbulence created at the valve can result in hemolysis (the rupture of individual blood cells) and thrombosis (the formation of blood clots), and it is now considered very important to avoid turbulence. Moreover, a prosthetic heart valve should be very responsive to the repetitive changes in the direction of blood flow, so as to quickly open during the pumping stroke and quickly close as soon as the heart relaxes the pumping chamber to thus prevent substantial regurgitation of blood. Furthermore, the opening and closing of such a heart valve should be sufficiently soft so as not to disturb the patient with audible sounds, and of course the heart valve should be constructed so as to withstand countless openings and closings.
U.S. Pat. No. 4,872,875, issued Oct. 10, 1989, discloses heart valve designs for both single occluder and bileaflet heart valves. The occluders are designed with a streamlined shape having an upstream generally flat section that leads to downstream section which vary in thickness; these downstream sections have surfaces which are three-dimensional in shape but which are formed with particular two-dimensional curved designs to eliminate boundary layer separation and thereby minimize the drag upon blood flow in the open position. Although such a design has substantial theoretical advantage, it is primarily based upon considerations applicable to surfaces of infinite length; because heart valve occluders are of finite length, edge effects become of considerable importance. Moreover there will no doubt always be areas for improvement in any heart valve design, and accordingly improved heart valve designs continue to be sought.